Some clients ask me whether a project can be cheaper if we cut the number of the slides, the answer is: not really. If your presentation designer is charging you by the slide, it means that she is likely to focus only on beautifying graphics page by page, rather than turning your entire story upside down and designing it from scratch.
Every presentation design project has a big fixed cost component: getting to know the client, getting to understand the story, setting up the overall look and feel of the presentation. After this, you need to put in however many slides it takes to tell the story, and I tend to err on putting in more than less. 30, 40, or even 50 slides, it does not make a lot of difference in the cost of a project.